A security system typically includes a security panel. The security panel includes an input/output (I/O) interface circuit for monitoring sensor circuits and controlling output circuits. The I/O interface circuit can include input interface circuits and output interface circuits.
Each input interface circuit monitors a sensor circuit. The sensor circuit can include a number of sensors that detect the same or different alarm conditions. Possible alarm conditions include opening of doors (door contact sensor/relay), excessive temperatures (heat sensor), motion (motion sensor), smoke (smoke sensor), and the like. The sensor circuit sends these alarm conditions (e.g., binary conditions or measured parameters) to the input interface circuit via an electrical connection such as one or more wires.
Each output interface circuit drives an output circuit. The output circuit can include one or more output devices to be activated on the occurrence of alarm conditions. Possible output devices may include sirens, lamps, relays, strobes, and the like. The output interface circuit controls each output device via a direct electrical (wire) connection between the output circuit and the output interface circuit.
Input and output requirements for the I/O interface circuit can vary from system to system, depending on system configuration, and particularly on the number of sensors and output devices. Some systems may have a large number of sensors but relatively few output devices thus requiring a larger number of input interface circuits as compared to output interface circuits. Other systems may have relatively few sensors but numerous output devices thus requiring a larger number of output interface circuits as compared to input interface circuits.
Driving the output circuit with the output interface circuit of a security panel can cause circuit damage to components of the output interface circuit. Damage can also arise when the output circuits and/or wire is compromised or ages. The circuit damage is due to overloading/overheating of the components. In particular, the circuit damage can occur from overloading conditions (e.g., power, current, and/or voltage levels exceed catalog limits for one or more components within the output interface circuit). Overloading conditions can be the result of an installer improperly connecting wires between the output circuit and output interface circuit, a faulty output device being used, wiring being damaged, or an old or damaged output device being used.
U.S. Pat. No. 8,891,217, incorporated in its entirety herein, provides one solution to the overloading problem. This publication generally describes an I/O interface circuit that monitors for overpower conditions based on power dissipation. In particular, the publication describes using a processing unit to monitor approximate power dissipated based on a measured voltage within the I/O interface circuit. If the measured voltage is indicative of excess power dissipation, one or more logic gates are used by the I/O interface circuit to prevent current flow. By preventing current flow, circuit damage due to excess power can be avoided.